The present invention relates to systems for designating targets by means of lasers and relates more specifically to a device for the alignment of the axes of the laser and the video image detector, called a sensor.
In such systems, a target is designated by means of a directional light beam obtained by a laser illuminator. The term pod is used to designate the fusiform member or nacelle, positioned externally of the carrying aircraft and which incorporates most of the system.
Advanced systems have automatic video tracking. Following designation of the target by means of the image sensor associated with the tracking device, the pointing of the laser beam on the target is automatically maintained without intervention by the pilot and independently of flight manoeuvres. When mounted in a pod adaptable to different types of apparatus, the system can be operated from a singleseater aircraft and is able to meet tactical requirements, particularly direct, low altitude, high speed attacks. The system makes it possible to guide rockets or missiles, like automatic laser guidance systems.
According to a prior art construction, such a system comprises a range-finder - laser illuminator functioning with coded pulses and with stabilization of the beam which remains pointed on the target by a television camera with automatic tracking. A gyroscope located at the front of the pod stabilizes the line of sight common to the television camera and the laser emission and permits the orientation thereof. The image of the scenery is reflected by a stabilized mirror towards the television camera across an optical path with a variable focal length. The television image is displayed in the cockpit and the video signals are processed by automatic tracking means. The laser illumination is reflected by a dichroic mirror placed in the optical path in such a way that it leaves parallel to the optical line of sight corresponding to the photograph of the scenery taken by the television camera. The stabilized mirror is used for isolating the line of sight from structural vibrations and permits the orientation thereof. Stabilization is produced by a yaw/pitch gyroscopic platform, whilst orientation is brought about by the rotation in roll of the front part synchronized with site and bearing movements of the mirror by a gimbal joint device. The television tracking system functions on two modes, the first serving to stabilize the image on the target area and to designate it and the second for automatic tracking operation. The first mode corresponds to the target acquisition or designation phase and the second can be produced by using the area correlation method or a video contrast analysis process.
One of the problems to be solved for these systems is the precise alignment throughout the designation phase of the axis of the laser emission with that of the sensor sighting field corresponding to the centre of the video image.
An automatic alignment method using the laser source is described in U.S. Pat. No. 4,155,096, which uses retroreflection of the laser radiation. However, this method has the following disadvantages.
The dichroic separator permits the passage of a fraction of the retroreflected laser energy to the image sensor and produces an image of the photosensitive surface thereof. The separator and the optical components positioned along the optical reception path leading to the sensor normally serve to transmit light energy having a wavelength differing from that of the laser and their transmittance at the wavelength of the laser source is often subject to considerable dispersion. It is therefore necessary to provide for these components specific characteristics for adapting to the wavelength of the laser. This also applies to the image sensitive T.V camera, FLIR or the like, which must have sensitivity characteristics at the wavelength of the laser.
Moreover, the laser is generally of the pulsed type, said pulses being supplied at a rate which is below the sweep frequency of the sensor. As a result the system circuits normally timed at the sweep frequency must be able to operate at the laser rate on the basis of video information. This is effective for automatic luminosity or video chain gain circuits and for the tracker.
Another problem occurs in the case where the laser source is coupled to a laser receiver for performing telemetry. The retroreflected laser energy following the dichroic separator essentially passes through the laser path, which represents a real risk for the laser receiver and makes it necessary to protect the latter during alignment.
Another difficulty can result from the constraints concerning the laser safety. Thus, the alignment cannot be performed in time before the target designation phase, because the laser is unusable during a large part of the mission in order to respect the safety regulations.
The object of the invention is to provide a device for the precise alignment of the laser and sensor axes and which does not have the disadvantages referred to hereinbefore. This largely results from the fact that the alignment method used does not employ the laser source. As a result, it is completely free from the indicated problems of the light wavelength, the laser rate, the laser energy return and the laser safety.